[cleanup] cleanup tests/[i18n_tests.py-isbn_tests.py]

• use single quotes for string literals
• remove preleading "u" fron strings
• indentation to make sure code lines are less than 79 characters
• use str.format(...) instead of modulo for type specifier arguments

Change-Id: If0d7074ad7d8ce65e903a26438a88ea8119fab9d